Paging systems have conventionally utilized a number of data links to transmit paging information from a central paging terminal to select pagers. Typically, paging information is coded with a unique identification number and then transmitted to all paging receivers, with the receivers only receiving information having a specific identification number associated therewith. However, paging transmitters can be located in any locale with the primary objective of a paging system being to deliver the signals to all paging receivers at virtually the same time in what is sometimes referred to as a Simultaneous Broadcast mode, or "simulcast".
In order to effect a simultaneous broadcast of the information, a radio link is provided with the information modulated onto a carrier and then the carrier transmitted to all the paging receivers. However, due to practical constraints, multiple paging transmitters are required at any given locale to provide sufficient coverage. Therefore, one problem that has been presented in the design of paging systems is that of independently transmitting from a plurality of transmitting antennas. It can be appreciated that when a receiver is proximate to one transmitter in an array of transmitters, the receiver will lock on to the stronger signal. However, a problem arises when a receiver is disposed equidistant between two transmitters with the power from each of the two transmitters being essentially the same magnitude. In this situation, data is being simultaneously received from two transmitters, and the phase difference between the data received from both transmitters must be relatively close. If not, an error may occur. It is accepted in the industry that any phase difference between two transmitters that would result in greater than one-half bit difference in the data received at a receiver midway between the two transmitters would cause an error in the reception of the data. Therefore, data transmitted from the two transmitters must be phased such that a smaller error is present.
In conventional systems, information is typically received at a paging terminal and then transmitted to the separate paging transmitters through either land lines or a radio control link. With land lines, there are delays inherent in the land line and, in the radio control links, delays occur as a result of path length, atmospheric conditions, etc. In each of these systems, some adjustment in phase must be made at the paging transmitter location to compensate for this phase. This is a conventional method, but does present problems in that the phase "drifts" over time as a function of degradation and parts, change in atmospheric conditions, etc. Therefore, the phase at each of the paging transmitters must be adjusted on a periodic basis.
In some conventional systems, the link between the paging terminal and the radio controlled link to the paging transmitters can be a satellite link. Typically, the paging terminal is connected to a satellite uplink which transmits the information to the satellite when a separate receiver is disposed elsewhere to receive the satellite information, decode it and modulate it onto the carrier of the radio control link for transmission to the paging transmitters. Typically, the radio control link is a "line of sight" link that has a large number of disadvantages with respect to terrain, atmospheric conditions, etc. Another disadvantage is that specific frequencies must be obtained from the appropriate regulatory bodies in order to utilize the radio controlled link. In crowded areas, these are difficult to obtain, due to the high demand for the various channels.